Train detector

ABSTRACT

A track monitoring circuit for detecting the presence of a train on a section of track by means of a pair of separated current transformers whose primary circuits are coupled to a portion of a common rail in either side of an audio frequency transmitter coupled across the track. The separation of the transformers is greater than the distance between the first and last wheels of one truck of a railway vehicle but less than the distance between the last wheel of the leading truck and the first wheel of a rear truck. The output of the transformers are converted respectively into first and second binary logic signals which are fed to first logic circuit means for providing an output signal indicative of the NOT AND function. The output of the first NOT AND logic circuit means is fed into two flip-flop circuits through a differentiator. Each flip-flop is respectively triggered by opposite edges of the output pulse of the first NOT AND logic gate. The outputs of these flip-flops are fed to a second NOT AND logic circuit means for providing an output which is adapted to activate suitable utilization or indicating means.

United States Patent I191 Pal [ Aug. 14, 1973 TRAIN DETECTOR [75]Inventor: Ajoy Kumar Pal, Downers Grove, Ill.

73 Ass ignee: Portec, Inc., Oak Brook, 111.

[22] Filed: May 7, 1971 [21] App1.No.: 141,270

[52] U.S. Cl. 246/249, 246/40 [51] Int. Cl B6ll 1/02 [58] Field ofSearch 246/34 CT, 40, 125-130, 246/34 R, 249

I 56] References Cited UNITED STATES PATENTS 3,333,096 7/1967 Ohman etal. 246/34 CT 3,471,689 10/1969 Wetmore 246/125 2,993,116 7/1961 Utt246/34 CT FOREIGN PATENTS OR APPLICATIONS 915,701

7/1954 Germany 246/34 R Primary Examiner-Gerald M. Forlenza AssistantExaminer-George I-l. Libman Attorney-Emory L. Groff and Emory L. Groff,Jr.

[5 7] ABSTRACT A track monitoring circuit for detecting the presence ofa train on a section of track by means of a pair of separated currenttransformers whose primary circuits are coupled to a portion of a commonrail in either side of an audio frequency transmitter coupled across thetrack. The separation of the tran sformers is greater than the distancebetween the first and last wheels of one truck of a railway vehicle butless than the distance between the last wheel of the leading truck andthe first wheel of a rear truck. The output of the transformers areconverted respectively into first and second binary logic signals whichare fed to first logic circuit means for providing an output signalindicative of the NOT AND function. The output of the first NOT ANDlogic circuit means is fed into two flip-flop circuits through adifferentiator. Each flip-flop is respectively triggered by oppositeedges of the output pulse of the first NOT AND logic gate. The outputsof these flip-flops are fed to a second NOT AND logic circuit means forproviding an output which is adapted to activate suitable utilization orindicating means.

9 Claims, 3 Drawing Figures l8 I numo FREQ. TRAIN 3| 2e TRANSMITTER IO24 29 RAIL gn L1 |e-l w 30 SIG/8 f? bzsl l4 -34 |2 32 L22 L ML BRIDGE AB BRIDGE R RECTIFIER RECTIFIER 0) L FLIP u. FLOP 66 TRAIN DETECTORBACKGROUND. OF THE INVENTION 1. Field of the Invention This inventionrelates to railway track apparatus and more particularly to means fordetecting occupancy of a predetermined stretch of railway track.

2. Description of the Prior Art The variousknown techniques fordetecting the presence of'a train along a stretch of railroad trackgenerally utilize either an audio frequency current applied directly tothe rails or an audio frequency current supplied to an AC bridge circuitembedded. in the track. The former'checks whether or not asufflcientcurrent is flowing through receiver units placed at each end of thedetectingzone and additionally includes electromechanical relays whichremain energized so long as the current flowing therethrough exceeds apredetermined threshold. These electromechanical relays are adapted tobecome. deenergized when car wheels and axles shunt the receivedterminals. The latter measures the inductance of a detector loop whichis embedded in the track. The detector loop is approximately-of the samelength-as the detector zone. Whenever a car or a'locomotiveapproachesthe detector loop, the inductance of the loop increases gradually. Thepresence of a. car or a locomotive will be indicated when the value ofthis inductance exceeds. a' predetermined threshold value.

The-limitation of the first technique Iies mainly in the also prone toprovide false indication inasmuch as.

other factors can influence the change in-the loop inductance apart fromthe presence of a railway vehicle.

SUMMARY I Recognizing the limitations of the prior art, the presentinvention discloses and claims an improved system for the detection of arailway vehicle in a predetermined tracksection. The inventioncomprises, inter alia, a pair of AC shunts coupled across the rails at afirst and-second point along the track. An audio frequency transmitteris coupled across the track rails intermediate the shunts and preferablyhalfway therebe tween, and applies a constant current, sinusoidal signalthereto. A first anda second current transformer has its primary windingcoupled in parallel to a portion of a rail at a track joint respectivelyon either side of the audio frequency transmitter. The primary windingis of such a nature that it will offer less resistance and inductivereactance to the audio frequency current transmitted along the rail. Theoutput winding of both transformers then provides an output-signal whichis dir'ectly proportional to the current flowing through the rail.

least equal to or greater than the distance between the first and lastwheels of one truck of a railway vehicle such as a locomotive or freightcar and less than the distance between the last wheel of the leadingtruck and the first wheel of the next or following truck. AC rectifiermeans are coupled to each secondary winding to respectively provide a DCoutput of a first magnitude when the wheels of a railway vehicle do notshunt the primarywinding and-a second magnitude when the primary windingis shunted thereby. Thus a binary logic v The separation between the twoprimary windings is at signal output is provided from each rectifierwhich is then coupled into a first binary logic circuit means providingthe NOT AND or NAND logic function. Two bistable or flip-flop circuitsare coupled to theoutput of the first logic circuit through adifferentiator. The respective outputs of the two flip-flops areconnected to a second NOT AND or NAND function circuit which provides anoutput pulse which control suitable circuit means coupled thereto forproviding either an indication of track occupancy or for operatingsuitable railway protection devices.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 isa block diagram illustrativeof a preferred embodiment of the subject invention;

FIG. 2is=a diagram-illustrating the relative separation distance betweentransformers in relation to the wheels of a railway vehicle; and

FIG. 3 is a timing diagram of voltage waveforms produced by theembodiment shown in FIG. I which are illustrative of the operation ofthe subject invention.-

DESCRIPTION OF THE PREFERRED EMBODIMENT Referringnow to the drawingsandmore particularly to FIG. 1, there is disclosed a portion of railwaytrackcomprised-of the rails 10 and 12 across which is connected apair ofAC shunts l4 and 16. The shunts 14 and 16. are both comprised of aseries circuit including an inductance and a capacitance which have aminimum value impedance at their series resonant frequency as-determinedby their respective component values. Intermediate the AC shunts l4 and16 and preferably midway therebetween is an audio frequency transmitter18 which is coupled across the rails 10 and 12 at the connections 20 and22. The resonant frequency-of the AC shunts l4 and 16 is chosen to besubstantially equal to the frequency of the audio frequency output fromthe transmitter 18 which comprises a sinusoidal substantially constantcurrent AC signal. The signal current for example coupled to the rail 10flows in the opposite-directions along the rail 10 through the shunts 14and 16, returning to the audio frequency transmitter 18 along rail 12.The maximum distance of the audio frequency current flow through therails 10 and 12, isthus limited by the position of the shunts l4 and 16on the track.

A first and a second current transformer" is coupled to the rail 10 oneither side of .the transmitter connection 20 within the track boundarydetermined by the shunts 14 and-16. The transformers 24 and 26 arelocated preferably substantially equal distances away from theconnection 20 and are preferably identical in configuration. The termcurrent transformer" is meant to designate that type of electricaltransformer wherein the voltage appearing across its secondary windingis proportional to the current flowing through its primary winding.Accordingly, the primary windings 28 and 30 of the transformers 24 and26, respectively, are connected'as parallel branch elements to the rail10 at the location of the joint sections 29 and 31 which may beinsulated or regular bar members. The primary windings are comprised ofsheet metal which exhibits relatively less resistance and inductivereactance to the audio frequency. currenttransmitted along the rail 10from the transmitter 18. Therefore substantially all of the current inthe two loops will flow through windings 28 and 30. To this end theprimary windings 28 and 30 are further illustrated as being single turnprimary windings having relatively less resistance to current flow thanthe rails themselves. The secondary windings 32 and 34 of thetransformers 24 and 26, respectively, are comprised of multi-turnwindings which are coupled into AC rectifiermeans 36 and 38 which areeach comprised of, for example, full-wave bridge rectifiers. Suchcircuits are well known to those skilled in the art and are comprised ofinterconnected solid state devices such as semiconductor diodes.

The separation of the current transformers 24 and 26 is designated by adistance d in FIG. 1 with the transmitter connections 20 and 22 beinglocated at the midpoint therebetween. Referring now briefly to FIG. 2,the distance d is selected to be at least equal to but preferablygreater than the distance d which is the distance between the first orleading wheels 40 and the rear wheels 42 of the leading truckof arailway vehicle 44 which may be for example a locomotive but less thanthe distance d which is the distance between the rear wheels 42 of theleading truck and the leading wheels 46 of the following truck. Thedistance d denotes the distance between the aforementioned trucks and isshown measured between the rear wheels 42 and 48.

Referring now back to FIG. 1, as long as no train occupies the sectionof track between the current transformer 24 and 26, the bridgerectifiers 36 and 38 provide a DC output of a constant or fixedmagnitude since the transmitter 18 is operating in a constant currentmode. However, whenever the wheels of a train pass the location ofeither of the transformers 24 or 26, the respective primary windings 28and 30 will be shunted by means of a short circuit provided by a pair ofwheels respectively in contact with the rails and 12 and the axle uponwhich the wheels are mounted. When this condition occurs the output ofthe respective bridge rectifier 36 and 38 falls to zero. Such an outputcorresponds to one of two possible binary logic states while thenon-shunted condition constitutes the other binary state. Accordingly, abinary output signal A appears on circuit lead 50 from the bridgerectifier means 38 which is coupled to one input of an AND circuit 52whose other input constitutes the binary output signal B from the bridgerectifier means 36 which appear on circuit lead 54. The AND gate 52 is awell known logic circuit and is often referred to as a coincidence gate.It operates to provide an output, for example, a binary 1 signal onlywhen the inputs A and B are also a binary l simultaneously, however, ifeither of the inputs is a binary 0, the output A'B will be a binary 0.The output of the AND gate 52 corresponding to the logic signal A-Bappears on circuit lead 56 and is applied to a logic inverter or NOTcircuit 58 which provides an output which is the complement of the logicfunctionA'B or NOT AB and which is expressed simply as H. Thecombination of the AND circuit 52 and inverter or NOT circuit 58 whendesirable can be configured as a single NAND logic gate 60 which is alsowell known to those skilled in the art.

The complementary output A 'B appears on circuit lead 62 and is appliedto a differentiator circuit 64 which provides an output comprised of twospikes or triggers signals of opposite polarities (positive andnegative). This output is applied to the diodes 66 and 68 which areconnected to bistable multivibrators or flipflop circuits 70 and 74respectively. Due to polarity connection of the diodes, flip-flop 70receives a positive trigger while flip-flop 74 receives a negativetrigger which cause the respective circuits to change state each time atrigger is coupled thereto. The outputs of these flip-flops designatedas Z and Z appear on circuit leads 72 and 76 and are fed to a seconddigital logic NOT AND circuit comprised of a second AND gate 78 whichprovides an output on the lead 80 corresponding to the Boolean functionZ 'Z and a NOT gate 82 which provides the complement. The signal NOT Z Zappearing on circuit lead 84 is adapted to control a relay 86 which inturn actuates a mechanical switch assembly, not shown, in the railroadclassification yard or a railroad crossing gate and/or warning indicatorapparatus.

The operation of the preferred embodiment of the subject invention shownin FIG. 1 can best be understood when considered in conjunction with thewaveforms shown in FIG. 3 and the relative distances shown in FIG. 2.Also a binary l is defined as the state of signals A or B whenever there.is a DC output of predetermined amplitude from the respective bridgecircuits 36 and 38 indicative of an unshorted primary winding. A binary0 designates a zero or no DC output which is indicative of a shortedprimary winding.

Normally when there is no railway vehicle within the detecting zonecorresponding to the distance d shown in FIGS. 1 and 2, currenttransformers 24 and 26 will be energized by an audio frequency AC signalcurrent transmitted from the transmitter 18 and consequently the outputsignals A and B from the bridge rectifiers 38 and 36, respectively, willexhibit a binary I state. The logic signal appearing at the output ofthe NOT circuit 58 is shown by waveform 88 of FIG. 3 and indicates thatit is in a binary 0" state. This corresponds to the time t Additionally,the output states of the flipflop circuits 70 and 74 as shown bywaveform 92 and 94 of FIG. 3 indicates that at the time t bothflip-flops are in a binary 1" state as opposed to a binary 0" state. Theflip-flop circuit 70 is responsive to the leading edge (positivetrigger) of the input signal 90 whereas the flip-flop 74 is responsiveto trailing edge (negative trigger) of the signal 90 and both changestates in accordance therewith. The signal 90 represents the output ofthe differentiator 64. The output of the logic inverter circuit 82 isillustrated by waveform 96.

The following truth table is helpful in understanding the operation ofthe logic circuitry as it pertains to the detection of a train withinthe zone defined by the transformers 24 and 26.

TABLE I TRUTH TABLE Condition 1 Condition 2 Condition 3 Condition 4Condition 5 Condition 6 Condition 7 Condition 8 Condition 9 When thezone between transformers 24 and 26 is unoccupied (condition 1 thesignals A and B are both I whereupon the logic function FD as referencedby waveform 88 shown in FIG. 3 is a 0. Waveforms 92 and 94 signifying ZandZ remain in a l state.

When a train enters the detection zone (condition 2) from left to rightas shown in FIGS. 1 and 2, the wheels 40 and axle not shown associatedtherewith shorts out the primary winding 30 whereupon signal A switchesto binary while signal B-remains in a binary l state. The signal A Ehowever, now is a binary l as indicated by waveform 88 at the time,tThis state will continue untilvthe wheels 40 reach the connections 20and 22 (condition 3) whereupon the signals A and B both become a-binary0. As the front wheels of the leading truck advance beyond theconnections 20-and 22 (condition 4), signal B will remain a logic 0;however, signal A'will become a binary 1 unless the rear wheels 42 ofthe leading-truck have in the meantime shorted primary winding 30,whereupon condition 2 and the 3 will again obtain. When both wheels 40and 42 of the leadingrtruck pass beyond the primary winding 28, signalsA and B will again revert to a. logic fl" as shown by condition-5 of thetruth table and which corresponds to the time t shown in FIG. 3.Condition 5 of the truth table will exist until the front wheels 46 ofthe following truck short out the primary winding 30 whereuponconditions 6 occurs, which is identical to'condition 2, and signal AB'a'gainassumes a binary 1 value until the rear truck'cle ars the zone(condition 9) whereupon the rear wheels 48 no longer short out theprimary winding 28. This occurs at the time 4 shown in FIG. 3.

Since the flip-flop circuits 70 and 74 are-respectively made responsiveto the leading and trailing edge of the ing zone to the right; whereasthe flip-flop 74 remains at this time in a l state. Consequently, Zwouldbe a binary *1 .f When the leading truck 'clears out the'dete'cting zone, the pulse A? drops down to 0" resulting in the'changeof state of the flip-flop'74 to a binary 0 state from a l-at time t,.Since the second pulse of waveform 88' isgenerated at time as the rear.truck including the wheels 46'and 48 travel over the detecting zone.The flip-flop 70 switches-to a the flip-flop 74 remains in a 0" stateand thereby the pulse Z will remain in a binary l The trailing edge ofthe second pulse corresponding tothe time t does not occur until therear wheel 48 of the second truck clears the primary winding 28. Atthis-time the-flip-flop circuit 74 is triggered back to its l state,whereas the flip-flop 70 is in a 1 state and-the pulse 2 will come backto fO as shown in the FIG. 3.

Thus an energizing signal is coupled-from the NOT gate 82 to,forexample, the relay 86 from a timewhen the first or leading truckenters the detectingzone after passing over the transformer 26 until therear wheels'of the second or rear truck have cleared the detecting zoneon the right of transformer 24. The set-reset" operations of theflip-flops 70 and 74 controlling the on-off switching-of the relay 86with the help of aNQT AND logic circuitry shown in FIG. 1 can beutilized not only for a railroad crossinggate activation system, but isequally adapted to be used in a railroad classification yard where itis" desirable to countthe number of train cars. This can easily be doneknowing the number of trucks in each car. I

,What has been shown and described, therefore, is a simple traindetection system utilizing digital logic circuitswhich are particularlyadapted to be fabricated state' whereas i from solid state devices whichwill providefar greater reliability than all other presently knowncircuits for such detection.

Having disclosed what is at present considered to ,be

the preferredtembodiment of the subject invention,

I claim as my invention: I w

1. Apparatus for detecting the presence of a railway vehicle withinapredetermined. track zone comprising in combination:

a section of track including a pair conducting electrical current;

first and second AC shunt means coupled across said rails and beingseparated a predetermined distance onsaid track section;

a source of alternating current of a selected frequency coupled acrosssaid rails intermediate said first and second shunt means;

a'first and second electrical transformer each having a primary windingand a secondary winding and includingmeans for coupling the primarywinding of said transformers in parallel with separate rail'portionsintermediate said first and second shunt means on opposite sides fromthe point of coupling of said source of AC current across said rails andbeing separated by a distance at least equal to the distance between thefirst and last wheels in one truck of a railway vehicle and less thanthe distance between the last wheel of the leading truck and thefirstwheel of a rear truck of said vehicle, said transformers beingadapted to provide an AC output signal across respective secondarywindings of a signal transmitted from said source but becomingdeenergized by the shorting effect of the train wheels across said railsanywhere between the respective primary winding and the point ofcoupling of said AC source to said rail;

first andsecond rectifier means respectively coupled of rails capable ofacrosssaid secondary winding of said first and second' transformerproviding a DC output of a first magnitude when the respective primarywinding is energized from said source and a DC output of asecondmagnitude when said primary winding is deenergized;

first NAND'digital logic circuit means having a pair of inputsrespectively coupled to the outputs A and B of said first and secondrectifier means and providing alogic output signal corresponding to F3;

a differentiator circuit coupled to the logic output signal of saidfirst NAND logic circuit means and providing a positive and a negativegoing trigger signal;

first and second bistable circuit means coupled to said differentiatorcircuit and including means for being respectively triggered by saidpositive .and negative going trigger signals to produce a respectivefirst and second square wave output signal Z and. Z

second NAND digital logic circuit means having a pair of inputsrespectively coupled to said first and second square wave output signalsof said bistable circuit means and providing a logic output signal m Zfor operating external utilization and indicating means.

, 2. The invention as defined by claim 1 wherein said primary winding ofsaid first and second transformer exhibits relatively less electricalresistance than the rail portion to which it is coupled in parallel soas to carry substantially most of the current flowing in the rail fromsaid source.

3. The invention as defined by claim 2 and wherein said secondarywinding of said first and second transformer provides an AC voltagethereacross which is proportional to the current flowing through saidrespective primary winding.

4. The invention defined by claim 3 wherein said source of AC currentcomprises an audio frequency transmitter operating in a constant currentmode.

5. The invention as defined by claim 1 and wherein said first and secondbistable circuit are comprised of flip-flop circuits.

6. The invention as defined by claim 1 wherein said track includes afirst and second rail joint respectively located on opposite sides ofthe coupling point of said source of AC current and wherein said firstand second primary winding are respectively coupled across said firstand second rail joint.

7. The apparatus as defined by claim 1 wherein said source ofalternating'current is coupled across said rails midway between saidfirst and second electrical trans former.

8. The apparatus as defined by claim 1 and additionally including firstand second diode means respectively coupled between aid differentiatorcircuit and said first and second bistable circuit meansand beingconductive in mutually opposite current directions to respectivelycouple said positive and negative trigger signals to said first andsecond bistable circuit means.

9. Apparatus as defined in claim 1 wherein said first and second NANDdigital logic circuit means each comprises a dual input AND logic gatecircuit and a logic inverter circuit coupled to the output of said ANDgate.

1. Apparatus for detecting the presence of a railway vehicle within apredetermined track zone comprising in combination: a section of trackincluding a pair of rails capable of conducting electrical current;first and second AC shunt means coupled across said rails and beingseparated a predetermined distance on said track section; a source ofalternating current of a selected frequency coupled across said railsintermediate said first and second shunt means; a first and secondelectrical transformer each having a primary winding and a secondarywinding and including means for coupling the primary winding of saidtransformers in parallel with separate rail portions intermediate saidfirst and second shunt means on opposite sides from the point ofcoupling of said source of AC current across said rails and beingseparated by a distance at least equal to the distance between the firstand last wheels in one truck of a railway vehicle and less than thedistance between the last wheel of the leading truck and the first wheelof a rear truck of said vehicle, said transformers being adapted toprovide an AC output signal across respective secondary windings of asignal transmitted from said source but becoming deenergized by theshorting effect of the train wheels across said rails anywhere betweenthe respective primary winding and the point of coupling of said ACsource to said rail; first and second rectifier means respectivelycoupled across said secondary winding of said first and secondtransformer providing a DC output of a first magnitude when therespective primary winding is energized from said source and a DC outputof a second magnitude when said primary winding is deenergized; firstNAND digital logic circuit means having a pair of inputs respectivelycoupled to the outputs A and B of said first and second rectifier meansand providing a logic output signal corresponding to A.B; adifferentiator circuit coupled to the logic output signal of said firstNAND logic circuit means and providing a positive and a negative goingtrigger signal; first and second bistable circuit means coupled to saiddifferentiator circuit and including means for being respectivelytriggered by said positive and negative going trigger signals to producea respective first and second square wave output signal ZF1 and ZF2;second NAND digital logic circuit means having a pair of inputsrespectively coupled to said first and second square wave output signalsof said bistable circuit means and providing a logic output signal ZF1 .ZF2 Zout for operating external utilization and indicating means.
 2. Theinvention as defined by claim 1 wherein said primary winding of saidfirst and second transformer exhibits relatively less electricalresistance than the rail portion to which it is coupled in parallel soas to carry substantially most of the current flowing in the rail fromsaid source.
 3. The invention as defined by claim 2 and wherein saidsecondary winding of said first and second transformer provides an ACvoltage thereacross which is proportional to the current flowing throughsaid respective primary winding.
 4. The invention defined by claim 3wherein said source of AC current comprises an audio frequencytransmitter operating in a constant current mode.
 5. The invention asdefined by claim 1 and wherein said first and second bistable circuiTare comprised of flip-flop circuits.
 6. The invention as defined byclaim 1 wherein said track includes a first and second rail jointrespectively located on opposite sides of the coupling point of saidsource of AC current and wherein said first and second primary windingare respectively coupled across said first and second rail joint.
 7. Theapparatus as defined by claim 1 wherein said source of alternatingcurrent is coupled across said rails midway between said first andsecond electrical transformer.
 8. The apparatus as defined by claim 1and additionally including first and second diode means respectivelycoupled between aid differentiator circuit and said first and secondbistable circuit means and being conductive in mutually opposite currentdirections to respectively couple said positive and negative triggersignals to said first and second bistable circuit means.
 9. Apparatus asdefined in claim 1 wherein said first and second NAND digital logiccircuit means each comprises a dual input AND logic gate circuit and alogic inverter circuit coupled to the output of said AND gate.